This invention relates to the field of film bulk acoustic resonators (FBAR), and in particular to FBAR's operable at microwave frequencies.
FBAR's are fabricated by sputter deposition of piezoelectric material, typically ZnO or AlN, on to a thin membrane formed on a semiconductor substrate. The combination of the piezoelectric layer and thin membrane forms an acoustic structure which is resonant at a specific frequency. A ZnO film having a thickness of a few microns will yield a resonator with a fundamental frequency of around 500 MHz.
A simple FBAR is illustrated in FIG. 1A, where a thin membrane of silicon dioxide 1 spans an opening in a semiconductor substrate 2. A layer of piezoelectric material 5 is disposed between input and output electrodes 7 and 9 respectively. Electrically, this device can be represented by the equivalent circuit shown in FIG. 1B, where L.sub.m, R.sub.m, and C.sub.m are motional parameters similar to those used with quartz crystals, and C.sub.o is the interelectrode capacitance. When this device is used as a series element in a filter, the interelectrode capacitance limits the attainable bandwidth. The interelectrode capacitance permits unwanted high-pass response, and when a parallel inductor is used as a neutralizing element, the parallel inductor permits an unwanted low-pass response.
An alternate structure which has none of these characteristics, and thus has the potential of higher frequency broadband operation, is the stacked FBAR, shown in FIG. 2A. The stacked FBAR is formed by adding a second layer of piezoelectric material to the standard FBAR, and by including a shield electrode. In FIG. 2A, a thin membrane of insulating material 20 spans an opening in a semiconductor substrate 22. In this device, input and output electrodes, 23 and 24 respectively, are separated by two layers of piezoelectric material, 25 and 26, and a shield electrode 27. Acoustically, this is essentially the same device as the standard FBAR of FIG. 1A, although an additional level of electromechanical coupling has been added. In this device, the shield electrode is connected to ground, providing isolation between the input and output electrodes, thereby essentially removing the interelectrode capacitance from consideration in filter design. This device is represented by the equivalent circuit shown in FIG. 2B, which utilizes the same symbology as FIG. 1B. When used as a series element in a filter, the stacked FBAR provides a true bandpass response.
Prior art FBAR devices are coupled to their associated circuitry by means of contact pads and wire bonds. This technique provides a poorly defined ground connection, because the inductance of the lead length lifts the actual ground away from true ground potential. While such prior art devices are adequate for lower frequencies applications, as the frequency climbs beyond 1 GHz, they rapidly become inoperable. A further disadvantage of the prior art devices is that the unwieldy transitions associated with the wire bond connections make simulation and computer analysis extremely difficult.